The present invention relates to the formation of semiconductor devices.
During semiconductor wafer processing, features of the semiconductor device are defined in the wafer using well-known patterning and etching processes. In these processes, a photoresist (PR) material is deposited on the wafer and then is exposed to light filtered by a reticle. The reticle is generally a glass plate that is patterned with exemplary feature geometries that block light from propagating through the reticle.
After passing through the reticle, the light contacts the surface of the photoresist material. The light changes the chemical composition of the photoresist material such that a developer can remove a portion of the photoresist material. In the case of positive photoresist materials, the exposed regions are removed, and in the case of negative photoresist materials, the unexposed regions are removed. Thereafter, the wafer is etched to remove the underlying material from the areas that are no longer protected by the photoresist material, and thereby define the desired features in the wafer.
Various generations of photoresist are known. Deep ultra violet (DUV) photoresist is exposed by 248 nm light. To facilitate understanding, FIG. 1A is a schematic cross-sectional view of a layer 108 over a substrate 104, with a patterned photoresist layer 112, over an ARL (Anti-reflective layer) 110 over the layer 108 to be etched forming a stack 100. The photoresist pattern has a critical dimension (CD), which may be the width 116 of the smallest feature. Presently, for 248 nm photoresist a typical CD for the photoresist may be 230-250 nm using conventional processes. Due to optical properties dependent on wavelength, photoresist exposed by longer wavelength light has larger theoretical minimal critical dimensions.
A feature 120 may then be etched through the photoresist pattern, as shown in FIG. 1B. Ideally, the CD of the feature (the width of the feature) is equal to the CD 116 of the feature in the photoresist 112. In practice, the CD of the feature 116 may be larger than the CD of the photoresist 112 due to faceting, erosion of the photoresist, or undercutting. The feature may also be tapered, where the CD of the feature is at least as great as the CD of the photoresist, but where the feature tapers to have a smaller width near the feature bottom. Such tapering may provide unreliable features.
In order to provide features with smaller CD, features formed using shorter wavelength light are being pursued. 193 nm photoresist is exposed by 193 nm light. Using phase shift reticles and other technology, a 90-100 nm CD photoresist pattern may be formed, using 193 nm photoresist. This would be able to provide a feature with a CD of 90-100 nm. 157 nm photoresist is exposed by 157 nm light. Using phase shift reticles and other technology sub 90 nm CD photoresist patterns may be formed. This would be able to provide a feature with a sub 90 mm CD.
The use of shorter wavelength photoresists may provide additional problems over photoresists using longer wavelengths. To obtain CD's close to the theoretical limit the lithography apparatus should be more precise, which would require more expensive lithography equipment. Presently 193 nm photoresist and 157 nm photoresist may not have selectivities as high as longer wavelength photoresists and may more easily deform under plasma etch conditions.
In the etching of conductive layers, such as in the formation of memory devices, it is desirable to increase device density without diminishing performance.